* Get rid of the substitutes database table (NIX-47). Instead, if we

need any info on substitutable paths, we just call the substituters
  (such as download-using-manifests.pl) directly.  This means that
  it's no longer necessary for nix-pull to register substitutes or for
  nix-channel to clear them, which makes those operations much faster
  (NIX-95).  Also, we don't have to worry about keeping nix-pull
  manifests (in /nix/var/nix/manifests) and the database in sync with
  each other.

  The downside is that there is some overhead in calling an external
  program to get the substitutes info.  For instance, "nix-env -qas"
  takes a bit longer.

  Abolishing the substitutes table also makes the logic in
  local-store.cc simpler, as we don't need to store info for invalid
  paths.  On the downside, you cannot do things like "nix-store -qR"
  on a substitutable but invalid path (but nobody did that anyway).

* Never catch interrupts (the Interrupted exception).

src/libutil/types.hh

@@ -19,23 +19,18 @@ using std::vector;
 using boost::format;
 
 
-class Error : public std::exception
+/* BaseError should generally not be caught, as it has Interrupted as
+   a subclass. Catch Error instead. */
+class BaseError : public std::exception 
 {
 protected:
     string err;
 public:
-    Error(const format & f);
-    ~Error() throw () { };
+    BaseError(const format & f);
+    ~BaseError() throw () { };
     const char * what() const throw () { return err.c_str(); }
     const string & msg() const throw () { return err; }
-    Error & addPrefix(const format & f);
-};
-
-class SysError : public Error
-{
-public:
-    int errNo;
-    SysError(const format & f);
+    BaseError & addPrefix(const format & f);
 };
 
 #define MakeError(newClass, superClass) \
@@ -45,6 +40,15 @@ public:
         newClass(const format & f) : superClass(f) { }; \
     };
 
+MakeError(Error, BaseError)
+
+class SysError : public Error
+{
+public:
+    int errNo;
+    SysError(const format & f);
+};
+
 
 typedef list<string> Strings;
 typedef set<string> StringSet;

src/libutil/util.cc

@@ -23,13 +23,13 @@ extern char * * environ;
 namespace nix {
 
 
-Error::Error(const format & f)
+BaseError::BaseError(const format & f)
 {
     err = f.str();
 }
 
 
-Error & Error::addPrefix(const format & f)
+BaseError & BaseError::addPrefix(const format & f)
 {
     err = f.str() + err;
     return *this;
@@ -491,6 +491,7 @@ string drainFD(int fd)
     string result;
     unsigned char buffer[4096];
     while (1) {
+        checkInterrupt();
         ssize_t rd = read(fd, buffer, sizeof buffer);
         if (rd == -1) {
             if (errno != EINTR)
@@ -775,6 +776,8 @@ void killUser(uid_t uid)
 
 string runProgram(Path program, bool searchPath, const Strings & args)
 {
+    checkInterrupt();
+    
     /* Create a pipe. */
     Pipe pipe;
     pipe.create();

src/libutil/util.hh

@@ -256,7 +256,7 @@ void inline checkInterrupt()
     if (_isInterrupted) _interrupted();
 }
 
-MakeError(Interrupted, Error)
+MakeError(Interrupted, BaseError)
 
 
 /* String packing / unpacking. */